Fabric sheets and nonwoven webs have been used as air filtration elements in dust-protective face masks for consumer and industrial use, respiratory protection devices, and medical devices (e.g., surgical, dental, medical procedure, isolation, and laser masks). Various types of face masks and respirators available to the general public and industrial workers are worn for protection against inhalation of dust, pollutants, allergens, pathogenic organisms, and other deleterious particulate materials.
Factors related to the protective efficiencies observed with face masks and respirators include the penetration efficiency and particle loading characteristics of the filtration materials. Also important is the fit of the face mask or respirator. Protection from dust, allergens, and infectious aerosols by face masks and respirators is dependent on the aerosol concentration of the compound and the infectious or inhaled dose. However, use of conventional face masks and respirators may not provide as much protection as desired against inhaled aerosols.
The filtration efficiency of nonwoven media can be influenced to some extent by the types of fibers used and their configuration. Nanofibrous media have been considered for their potential to improve the filtration efficiency over that of media made with larger fibers, because a smaller pore size might be expected from use of fibers with smaller diameter. On other hand, another measure of filter performance is the resistance or pressure drop, which ordinarily increases as pore size decreases. A higher resistance makes face masks less breathable and less comfortable.
In addition, known nanofiber media are typically soft and fragile, so they are not self-supporting and thus cannot be used alone as air filtration elements. Thus, present nanofibers are typically coated or otherwise supported on a more robust substrate to form a composite that can be handled readily in the course of manufacturing, transport, and use. The substrate is commonly a separate, non-woven microfiber medium. Nanofibers used for the coating can be produced by either electrospinning or melt-blown processes. Although electrospinning in some cases can produce small diameter fibers, the production rate is ordinarily so low that the ultimate cost is prohibitive. Melt blown nanofibers are also relatively expensive when compared to standard filter media. Even islands-in-the-sea nanofibers, which can be produced at high rates, are costly to produce because they require a removable sea that must be removed in a separate process step. Melt blown nanofiber processes or melt film fibrillation process that randomly lay down fibers do not provide adequate uniformity at sufficiently high throughputs for most end use applications.
Nanofibers have been considered for use in face masks, e.g. as a coating layer on a substrate or laminated with a substrate or as a nanofiber layer structure. Although the resulting nanofiber webs in some instances have good filtration efficiency, they typically exhibit a very high resistance (pressure drop) which signals poor breathability and less comfort for the mask wearer.
What is needed is a low-cost filtration media that can be incorporated in face masks, respirators, and similar personal protection devices that can efficiently trap tiny particles, while providing desirable breathability and user comfort. Ideally, protective equipment constructed with such media would be suitable for filtering ambient bacteria, pollen, virus-containing small particle aerosols, and dusts emitted by industrial processes or from other natural sources.